A gas turbine engine of the type adapted for use in an aircraft is one example of a rotary machine having a turbine section. The turbine section is disposed about an axis of symmetry R of the engine. The engine has a compression section and a combustion section upstream of the turbine section which are also disposed about the axis R. An annular flowpath for working medium gases extends axially through the compression section, the combustion section and the turbine section of the engine.
The working medium gases are compressed in the compression section and expanded in the turbine section. As the gases are flowed through the compression section, the gases encounter an increasing or adverse pressure gradient. In the turbine, the gases encounter a decreasing or favorable pressure gradient. These differences in pressure gradients cause the aerodynamic considerations of the turbine to vary greatly from the aerodynamic considerations of the compression section.
Fuel is mixed with the working medium gases in the combustion section and burned to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to develop propulsive thrust and, through one or more of the turbines, to extract energy from the gases by driving the turbines about an axis of rotation A.sub.r of the engine.
The turbine section is adapted by a rotor assembly to extract work from the gases as the working medium gases are expanded through the turbine section. The rotor assembly typically includes one or more rotor disks and arrays of flow directing elements in the form of rotor blades which extend outwardly from the disk. The rotor blades have airfoils which extend across the working medium flowpath to guide the gases through the rotor assembly and to develop a driving force for the rotor assembly by flowing the gases over an airfoil shaped contour on the rotor blade.
To accomplish this purpose, the airfoil section of the rotor blade has a concave or pressure side surface which extends axially rearwardly from the leading edge of the rotor blade to the trailing edge. A convex or suction side surface extends rearwardly from the rotor blade and is joined to the concave surface at the trailing edge of the rotor blade. The difference in velocity of the gases as the gases pass through the array of rotor blades and over the airfoils causes a difference in static pressure across the airfoil (or lift) resulting in a force. The force acts on the airfoil, driving the rotor assembly about its axis of rotation.
Aerodynamic losses occur as the gases pass through the array of rotor blades and over the airfoils of the turbine. The loss in energy associated with these aerodynamic losses, decreases the efficiency of the turbine and thus decreases the efficiency of the engine. Accordingly, a great deal of effort has been devoted to decreasing these aerodynamic losses.
Because significant losses are associated on the suction side surface with separation of the flow from the airfoil, it has been suggested by many sources that the suction surface of the airfoil be roughened to avoid separation. One example of such a reference is G.B. Patent 580,806 issued to Griffith entitled "Improvements In Compressor, Turbine And Like Blades" which was filed in 1941. Griffith suggests that doing so has particular advantages when applied to the blades of compressors, but is of value in some conditions of operation in a turbine. However, roughening the surface introduces stress concentrations. this is of concern in the turbine because of the stresses which the blade encounters in high temperature environment of the turbine.
Another approach is to apply a local trip device to the blade of the turbine as suggested in U.S. Pat. No. 4,822,249 issued to Eckardt entitled "Axial Flow Blade Wheel Of A Gas Or Steam Driven Turbine". In Eckardt, the local projection might be a spoiler edge of sawtooth profile which disrupts the velocity profile and thus the pressure distribution of the airfoil to affect separation and avoid the formation of laminar separation bubbles. Still other approaches to avoid separation are to energize the boundary layer by the injection of fluid or to prevent separation by creating a suction on the interior of the airfoil such that the boundary layer is pulled inwardly to the interior of the blade.
Still another approach to improving aerodynamic performance has been to polish the surfaces of the airfoil, thereby removing the roughness of the airfoil which results from polishing the airfoil. This was the approach used most recently for modern gas turbine engines manufactured by The Pratt & Whitney Group of Applicant's Assignee. One advantage of removing roughness from the blades is that surface roughness introduces stress concentrations into the blade.
In such applications, the finished blade had a surface roughness after fabrication by casting of 60 AA-80 AA microinches. The blade was finished by polishing operations to reduce the roughness of the surface to 30 AA microinches (commonly referred to as the surface roughness of "30 AA").
In this regard, it is important to designate the type of measurement used to evaluate surface roughness because many standards exit and because of the difficulty in measuring surface irregularities, which are very complex in shape and character. The "AA" designation of surface-texture quality is a commonly accepted standard, as pointed out in Mark's Handbook Of Mechanical Engineering (9th edition--1987). This standard is set forth in the ANSI B46.1 standard. Marks notes the ANSI B46.1 standard "conforms in all essential elements with the British, Canadian and most ISO international standards, even though different terms are used [to designate them]; i.e., the R.sub.a, the AA (arithmetic average), and the CLA (centerline average) are identical with the internationally adopted symbol of R.sub.a of ISO R468". In this standard, the height rating is expressed as an average deviation from the mean surface. The mean surface is a surface located in such a way that the volume of the peaks above the mean surface is equal to the volume of the valleys below the mean surface. The mean surface is the perfect surface that would result if all the peaks were leveled off and the material used to fill the valleys. The arithmetic average deviation of the surface of the irregularities from the mean surface is defined as the result of taking a great many uniformly spaced measurements in microinches and averaging them.
The above art not withstanding, scientists and engineers working under the direction of Applicant's Assignee have sought to decrease flow losses through turbine cascades in a way which might change the surface of the airfoil, while still having an airfoil with acceptable stress concentrations at critical locations in the airfoil for acceptable fatigue life of the airfoil.